Synopsis On 08 June 1994, the FEDERAL MACKENZIE, en route from Chicago, USA, was transiting in the Seaway toward Dalhousie, New Brunswick. At about 1150(1), the vessel entered the St. Lambert Lock in Quebec, with a pilot on board, in light condition and at a speed of about 1.25 knots. The vessel moved normally into the lock until she reached the 25-metre mark, located 29.6 metres from the ship arrester, at which time the bridge control gave the order to reverse the propeller pitch. A few seconds later, the propeller was moved to half astern and, when the vessel reached the 10-metre mark, which is 14.6 metres from the ship arrester, the order for full astern was given. The manoeuvre did not occur as planned; the vessel struck ship arrester No. 2 and deflected it about four feet. The vessel was not damaged and no injuries were reported. Following this accident, the St. Lambert Lock was closed temporarily for repairs to the ship arrester. The repairs delayed four other vessels moving through the Seaway for about 12 hours. Ce rapport est galement disponible en franais. The dimensions of the FEDERAL MACKENZIE correspond to the maximum dimensions for vessels using the Seaway. A test of the electronic circuits revealed a failure in the module used to simultaneously control the propeller pitch mechanism and the engine rpm when the system is actuated from the bridge control. Thus, when the propeller-pitch control lever was moved to reverse, the system did not respond up to the percentage ordered by the bridge control. This resulted in a slow speed astern, which was clearly not enough to bring the vessel to a stop and to prevent the striking of ship arrester No. 2.